Acid-base mixture and ion conductor comprising the same

ABSTRACT

Disclosed is an acid-base mixture composed of a base component and an acid component, wherein at least one of the base component and the acid component contains at least two compounds, and the base component contains at least one compound of formula (1):  
                 
 
wherein R 1 , R 2 , and R 3  each represent a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, provided that at least one of them is a hydrocarbon group. Also disclosed is an ion conductor comprising an acid-base mixture composed of an acid component and a base component including at least one compound of formula (2):  
                 
 
wherein R 1 , R 2 , and R 3  each represent a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, provided that R 1  and R 3  are different.

TECHNICAL FIELD

The present invention relates to a mixture of a base component and anacid component and an ion conductor comprising the same. The ionconductor of the present invention is useful in fuel cells, secondarybatteries, electric double layer capacitors, electrolytic capacitors,etc.

BACKGROUND ART

Imidazolium salts are well known to be useful as an epoxy resin curingagent. While most of imidazolium salts are solid, JP-A-57-190018 (PatentDocument 1) discloses a 2-ethylhexanoate or an acetate of an imidazolecompound as an epoxy resin cure accelerator that is liquid at roomtemperature. Journal of Japan Society of Colour Material, 50 (1), 2-7(1977) (Non-Patent Document 1) teaches that an imidazole compound saltwith an alkylcarboxylic acid or a phosphoric acid is liquid at roomtemperature and reports epoxy resin curing by the use of the salt.JP-A-48-5900 (Patent Document 2) discloses an epoxy resin compositioncontaining a sulfonate of an imidazole compound as a curing agent or acure accelerator. U.S. Pat. No. 3,356,645 (Patent Document 3) disclosesa carboxylate, a lactate, and a phosphate of an imidazole compound. Allthe references cited above neither describe nor suggest ion conductivityof these salts.

Some of ammonium salts such as imidazolium salts and pyridium salts areknown to become a liquid molten salt at or below 100° C., particularlyaround room temperature and to exhibit high ion conductivity atrelatively low temperatures of 200° C. or lower without using water oran organic solvent. Such molten salts have been studied forapplicability as an electrolyte of batteries and the like for theircharacteristic nonvolatility. Known ionic liquids include a number ofimidazole salts or pyridine salts having a substituent introduced totheir N-position(s) (see Hiroyuki Ohno (ed.), Ionsei Ekitai—Kaihatsu noSaizensen to Mirai-, CMC Publishing Co., Ltd., 28-31 (2003): Non-PatentDocument 2).

Watanabe, et al. report protic, room-temperature molten salts in J.Phys. Chem. B., 107 (17), 4024-4030 (2003) (Non-Patent Document 3),Chem. Commun., 938-939 (2003) (Non-Patent Document 4), Proceedings ofThe 43rd Battery Symposium in Japan, 102-103 (2002) (Non-Patent Document5), and ibid., 604-605 (2002) (Non-Patent Document 6). The reportedprotic, room-temperature molten salts are prepared basically using anamine compound with the positions other than the N-positionunsubstituted.

Kreuer, et al. report a proton conductor composed of unsubstitutedimidazole and sulfuric acid in Electrochimica Acta, Vol. 43, No. 10-11,1281-1288 (1998) (Non-Patent Document 7). JP-T-2000-517462 (PatentDocument 4) discloses a proton conductor containing an acid and anonaqueous amphoteric material. The imidazole compounds having asubstituent at a position other than the 1,3-positions which aredisclosed therein are given as a general formula having one substituentat such a position. The description is absent on limitation of theposition of the substituent. The imidazole compound actually used in thedescription is an unsubstituted compound.

Armand et al. (JP-T-2000-508114: Patent Document 5) discloses a protonconductor in liquid form comprising a mixture of an acid addition saltof a nitrogen base and a nitrogen base, wherein the acid of the acidaddition salt is a fluorine-based acid. The fluorine-based acid involvescost and environmental concerns in the production. Use of the basiccomponent (nitrogen base) in excess lowers the melting point but reducesheat resistance.

-   Patent Document 1: JP-A-57-190018-   Patent Document 2: JP-A-48-5900-   Patent Document 3: U.S. Pat. No. 3,356,645-   Patent Document 4: JP-T-2000-517462-   Patent Document 5: JP-T-2000-508114-   Non-Patent Document 1: Journal of Japan Society of Colour Material,    50 (1), 2-7 (1977)-   Non-Patent Document 2: Ionsei Ekitai—Kaihatsu no Saizensen to    Mirai-, CMC Publishing Co., Ltd., 28-31 (2003)-   Non-Patent Document 3: J. Phys. Chem. B., 107 (17), 4024-4030 (2003)-   Non-Patent Document 4: Chem. Commun., 938-939 (2003)-   Non-Patent Document 5: Proceedings of The 43rd Battery Symposium in    Japan, 102-103 (2002)-   Non-Patent Document 6: Proceedings of The 43rd Battery Symposium in    Japan, 604-605 (2002)-   Non-Patent Document 7: Electrochimica Acta, Vol. 43, No. 10-11,    1281-1288 (1998)

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an ion conductiveacid-base mixture having a relatively low melting point and an ion orproton conductor containing the mixture.

The present invention accomplishes the above object by providing anacid-base mixture composed of a base component and an acid component andan ion or proton conductor containing the mixture. At least one of thebase component and the acid component includes at least two compounds.The base component contains at least one compound represented bychemical formula (1) shown below (The acid-base mixture will hereinafterbe referred to as “the acid-base mixture of the first aspect).

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that at least one of them isa hydrocarbon group.

The present invention also accomplishes the above object by providing anion conductor comprising an acid-base mixture composed of a basecomponent containing a base represented by chemical formula (2) shownbelow and an acid component (The ion conductor will hereinafter bereferred to as the ion conductor of the second aspect).

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that R¹ and R³ aredifferent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the temperature dependence of the ionconductivity of the acid-base mixtures prepared in Examples 1, 2, 6, 7,and 8 and Comparative Example 1.

FIG. 2 is a graph showing the results of thermogravimetric analysis onthe acid-base mixtures of Examples 1, 7, and 8.

FIG. 3 is a graph showing the results of thermogravimetric analysis onthe acid-base mixtures of Examples 2 and 3 and Comparative Example 1.

FIG. 4 is a graph showing the temperature dependence of the ionconductivity of the acid-base mixtures of Examples 6, 15, and 16 andComparative Example 1.

FIG. 5 is a graph showing the temperature dependence of the ionconductivity of the acid-base mixtures of Examples 7 and 8 andComparative Example 1.

FIG. 6 is a graph showing the results of thermogravimetric analysis onthe acid-base mixtures of Examples 6, 21, 22, and 24, 2E4MZ, andmethanesulfonic acid.

FIG. 7 is a graph showing the results of thermogravimetric analysis onthe acid-base mixtures of Examples 7, 10, and 11.

FIG. 8 is a graph showing the results of thermogravimetric analysis onthe acid-base mixtures of Examples 7 and 8 and Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The acid-base mixture of the first aspect will be described first.

The acid-base mixture of the first aspect is composed of a basecomponent and an acid component. At least one of the base component andthe acid component includes at least two compounds. The base componentcontains at least one base represented by chemical formula (1) shownbelow, preferably a base represented by chemical formula (2) shownbelow.

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that at least one of them isa hydrocarbon group.

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that R¹ and R³ aredifferent.

Preferred examples of the hydrocarbon group having 1 to 20 carbon atomsinclude straight-chain or branched alkyl groups and aromatic groups.Specific examples are methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl, tert-butyl, hexyl, phenyl, and benzyl, with methyland ethyl being particularly preferred.

The bases represented by chemical formulae (1) or (2) include imidazoleshaving an alkyl group at a position other than the N-positions of thering, such as monoalkylimidazoles, e.g., 2-alkylimidazoles and4-alkylimidazoles, and 2,4-dialkylimidazoles.

Specific examples of the above-described bases include2-alkylimidazoles, such as 2-methylimidazole and 2-ethylimidazole;4-alkylimidazoles, such as 4-methylimidazole and 4-ethylimidazole;2,4-dialkylimidazoles, such as 2-ethyl-4-methylimidazole,2-octyl-4-hexylimidazole, 2-cyclohexyl-4-methylimidazole, and2-butyl-4-allylimidazole; 2-phenylimidazole, 4-phenylimidazole, and2-ethyl-4-phenylimidazole.

Preferred of them are 2-ethyl-4-methylimidazole, 4-methylimidazole, and2-ethylimidazole.

Where the acid component is a single compound, the base component is amixture of two or more of the above-described bases. Where the acidcomponent is a mixture of two or more compounds, the base component maybe either a single compound or a mixture of two or more compoundsselected from the above-described bases. Where the base component is amixture of two or more compounds, one of them may be unsubstitutedimidazole. In this case, the proportion of unsubstituted imidazole inthe base component is preferably 90% by weight or less, still preferably60% by weight or less.

Suitable combinations of two or more bases include a mixture of2-ethyl-4-methylimidazole and 4-methylimidazole, a mixture of2-ethyl-4-methylimidazole and 2-ethylimidazole, a mixture of2-ethyl-4-methylimidazole and imidazole, and a mixture of2-ethylimidazole and 4-methylimidazole.

The acid component that can be used in the first aspect of the inventionincludes sulfonic acids, sulfonic acid compounds, carboxylic acids, andinorganic acids. It is preferred that at least one compound making upthe acid component be an inorganic acid. Examples of the acid componentinclude organic aliphatic or aromatic sulfonic acids, such asp-toluenesulfonic acid, methanesulfonic acid, andtrifluoromethanesulfonic acid; and aromatic or aliphatic carboxylicacids. Preferred inorganic acids include inorganic mineral acids, suchas sulfuric acid, phosphoric acid, and perchloric acid. The acidcomponent is preferably an acid containing no fluorine atom in itsstructure. Sulfuric acid, phosphoric acid, etc. are advantageous fromthe standpoint of cost. Methanesulfonic acid, etc. are advantageous fromthe standpoint of ease of handling.

Where the base component is a mixture of two or more bases, the acidcomponent may be either one or a mixture of two or more selected fromthe above-recited acids. Where the base component is a single compound,the acid component should be a mixture of two or more of theabove-described acids.

Suitable combinations of the acid component and the base componentinclude: a combination of 2-ethyl-4-methylimidazole/4-methylimidazoleand sulfuric acid (2E4MZ/4MI.H₂SO₄), a combination of2-ethyl-4-methylimidazole/2-ethylimidazole and sulfuric acid(2E4MZ/2EI.H₂SO₄), a combination of 2-ethyl-4-methylimidazole/imidazoleand sulfuric acid (2E4MZ/Im.H₂SO₄), and a combination of2-ethylimidazole/4-methylimidazole and sulfuric acid (2EI/4MI.H₂SO₄).

The mixing ratio of the base component to the acid component preferablyranges from 99:1 to 1:99, still preferably from 95:1 to 1:95, by mole.It is not preffered that the ratio of the base component or the acidcomponent exceeds the recited range because the heat resistance reduces.A particularly preferred mixing ratio of the base component and the acidcomponent is 1:1 (an equimolar mixture).

The acid-base mixture of the first aspect is preferably an acid-basemixture having a melting point of 120° C. or lower or a liquid acid-basemixture showing no melting point.

It is particularly preferred that the acid-base mixture of the firstaspect be an acid-base mixture that is liquid at room temperature or anacid-base mixture having a glass transition temperature of 25° C. orlower.

The acid-base mixture of the first aspect is preferably an ion conductorhaving an ion conductivity of, for example, 10⁻⁴ Scm⁻¹ or higher at 100°C. The acid-base mixture of the first aspect is superior in ionconductivity in a low temperature region of room temperature or lower.

The ion conductor according to the second aspect of the invention willthen be described.

The ion conductor of the second aspect comprises an acid-base mixturecomposed of a base component containing a base represented by chemicalformula (2) shown below and an acid component.

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that R¹ and R³ aredifferent.

Preferred examples of the hydrocarbon group having 1 to 20 carbon atomsinclude straight-chain or branched alkyl groups and aromatic groups.Specific examples are methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl, tert-butyl, hexyl, phenyl, and benzyl, with methyland ethyl being particularly preferred.

The base represented by chemical formula (2) used in the second aspectis an asymmetric imidazole compound in which the substituents R¹ and R³are not the same.

Such asymmetric imidazole compounds include monoalkylimidazoles havingone alkyl group at a position other than the N-positions of the ring,such as 4-alkylimidazoles.

Specific examples include 4-alkylimidazoles such as 4-methylimidazoleand 4-ethylimidazole, and 4-phenylimidazole.

The asymmetric imidazole compounds further include those having an alkylgroup at two or more positions other than the N-positions of the ring,such as 2,4-dialkylimidazoles.

Specific examples include 2,4-dialkylimidazoles such as2-ethyl-4-methylimidazole, 2-octyl-4-hexylimidazole,2-cyclohexyl-4-methylimidazole, and 2-butyl-4-allylimidazole, and2-ethyl-4-phenylimidazole.

Of the asymmetric imidazole compounds described above, preferred are4-methylimidazole and 2-ethyl-4-methylimidazole.

The base component used in the second aspect may be either one of theabove-described bases or a mixture of two or more of them. Where amixture of two or more bases is used, one of them may be unsubstitutedimidazole or a symmetric imidazole compound such as a 2-alkylimidazoleor 2-phenylimidazole. Examples of the 2-alkylimidazole are2-methylimidazole and 2-ethylimidazole. In this case, the proportion ofthe unsubstituted imidazole or the symmetric imidazole compound in thebase component is preferably 90% by weight or less, still preferably 60%by weight or less.

Suitable combinations of the two or more bases include a mixture of2-ethyl-4-methylimidazole and 4-methylimidazole, a mixture of2-ethyl-4-methylimidazole and 2-ethylimidazole, a mixture of2-ethyl-4-methylimidazole and imidazole, and a mixture of4-methylimidazole and 2-ethylimidazole.

The acid component that can be used in the second aspect includes theacids described for use in the first aspect of the invention, which canbe used either individually or as a mixture of two or more thereof.

Suitable combinations of the acid component and the base componentinclude: a combination of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄), a combination of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf), a combination of4-methylimidazole and sulfuric acid (4MI.H₂SO₄), a combination of2-ethyl-4-methylimidazole/4-methylimidazole and sulfuric acid(2E4MZ/4MI.H₂SO₄), a combination of2-ethyl-4-methylimidazole/2-ethylimidazole and sulfuric acid(2E4MZ/2EI.H₂SO₄), a combination of 2-ethyl-4-methylimidazole/imidazoleand sulfuric acid (2E4MZ/Im.H₂SO₄), a combination4-methylimidazole/2-ethylimidazole and sulfuric acid (4MI/2EI.H₂SO₄),and a combination of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H).

The mixing ratio of the base component to the acid component preferablyranges from 99:1 to 1:99, still preferably from 95:1 to 1:95, by mole.If the ratio of the base component or the acid component exceeds therecited range, the heat resistance reduces. A particularly preferredmixing ratio of the base component and the acid component is 1:1 (anequimolar mixture).

The ion conductor of the second aspect contains the acid-base mixturecomposed of the base component and the acid component and exhibits anion conductivity of, for example, 10⁻⁴ Scm⁻¹ or higher at 100° C. Theion conductor of the second aspect is superior in ion conductivity in alow temperature region of room temperature or lower.

The ion conductor of the second aspect is preferably one having amelting point of 120° C. or lower or a liquid one with no melting point.

It is particularly preferred for the ion conductor of the second aspectto have a glass transition temperature of 25° C. or lower.

EXAMPLES

The present invention will now be illustrated in greater detail withreference to Examples and Comparative Examples. Measurements in Examplesand Comparative Examples were made in accordance with the followingmethods.

(1) Measurement of Ion Conductivity

A dried sample was put in a sample bottle. Platinum plates measuring 1.5cm wide and 2 cm long were immersed in the sample in parallel with eachother at a 1 cm distance. The sample bottle was closed to make a cellfor conductivity measurement. The ionic conductivity was obtained bycomplex impedance measurement with FRD 1025 and Potentiostat/Galvanostat283, supplied by Princeton Applied Research, in a thermostat set at aprescribed temperature.

(2) Melting Point

Measured with DSC-7 from Perkin-Elmer Inc. or DSC-50 from Shimadzu Corp.at a rate of temperature rise of 10° C./min in a helium stream.

(3) Thermogravimetry

Carried out with TGA-50 from Shimadzu Corp. in air at a rate oftemperature rise of 10° C./min.

Example 1 Mixture of 2-ethyl-4-methylimidazole/4-methylimidazole andsulfuric acid (2E4MZ/4MI.H₂SO₄; molar ratio=1:1:2)

In 12.7 g of 2E4MZ (from Shikoku Chemicals Corp.) was added dropwise 6ml of 98% sulfuric acid in a nitrogen atmosphere while stirring. After 2hour stirring, 20.5 g of 4MI.H₂SO₄ prepared in Example 8 given later wasadded thereto, followed by stirring overnight at room temperature. Themixture was dried under reduced pressure at 110° C. for 6 hours toremove water to give 2E4MZ/4MI.H₂SO₄ (molar ratio=1:1:2). The acid-basemixture maintained the liquid state for more than 4 months. The resultsof DSC showed no melting point and a Tg of −54° C. The temperaturedependence of the ion conductivity of the acid-base mixture is shown inFIG. 1.

Example 2 Mixture of 2-ethyl-4-methylimidazole/2-ethylimidazole andsulfuric acid (2E4MZ/2EI.H₂SO₄; molar ratio=1:1:2)

In a flask were put 15.7 g of 2E4MZ and 13.7 g of 2EI (from Aldrich),and the 2EI was melted at 100° C. to make a uniform mixture. Into themixture was added dropwise 15 ml of 98% sulfuric acid in a nitrogenatmosphere while stirring. The mixture was stirred at room temperatureovernight, followed by drying under reduced pressure at 110° C. for 6hours to remove water thereby to give 2E4MZ/2EI.H₂SO₄ (molarratio=1:1:2). The resulting acid-base mixture maintained the liquidstate for more than 5 months. The DSC results revealed no melting pointand a Tg of −61° C. The temperature dependence of the ion conductivityof the acid-base mixture is shown in FIG. 1. Owing to the mixed basesystem, the acid-base mixture of Example 2 exhibited improvement in ionconductivity in a low temperature region over the acid-base mixture ofComparative Example 1 hereinafter given.

Example 3 Mixture of 2-ethyl-4-methylimidazole/imidazole and sulfuricacid (2E4MZ/Im.H₂SO₄; molar ratio=1:1:2)

In 30 ml of ethanol were dissolved 5.17 g of 2E4MZ and 3.20 g ofimidazole (from Sigma). The solution was cooled in an ice bath, and 5 mlof 98% sulfuric acid was added thereto dropwise in a nitrogen atmospherewhile stirring. The stirring was continued at room temperatureovernight, followed by drying under reduced pressure at 60° C. for 1hour and then at 110° C. for 6 hours to remove ethanol and water to give2E4MZ/Im.H₂SO₄ at a molar ratio of 1:1:2. The resulting acid-basemixture was solid at room temperature. In DSC, a sample was maintainedat 100° C. to once melt, cooled to −150° C., and again heated from −150°C. up to 100° C. The sample showed only a Tg with no peak ofcrystallization or melting in both the cooling and the heatingthermograms. The Tg was −56° C.

Example 4 Mixture of 2-ethylimidazole/4-methylimidazole and sulfuricacid (2EI/4MI.H₂SO₄; molar ratio=1:1:2)

A mixture of 4.51 g of 2EI and 3.85 g of 4MI (from Aldrich) was meltedat 100° C., and 5 ml of 98% sulfuric acid was added thereto dropwise ina nitrogen atmosphere while stirring. After the mixture was stirred atroom temperature overnight, water was removed by drying under reducedpressure at 110° C. for 6 hours to give 2EI/4MI.H₂SO₄ (molarratio=1:1:2). The acid-base mixture maintained the liquid state for morethan 3 months.

Example 5 Mixture of 2-ethyl-4-methylimidazole/4-methylimidazole andsulfuric acid (2E4MZ/4MI.H₂SO₄; molar ratio=1:1:1)

In a flask were put 10.5 g of 2E4MZ and 7.70 g of 4MI, and the 4MI wasmelted at 70° C. to make a uniform mixture. To the mixture was addeddropwise 5 ml of 98% sulfuric acid in a nitrogen atmosphere whilestirring. After a while, the viscosity increased to make stirringdifficult. From the next day on, the resulting mixture 2E4MZ/4MI.H₂SO₄having a molar ratio of 1:1:1 gradually solidified. It completelysolidified in three months.

Example 6 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf)

In 50 ml of ethanol was dissolved 62.3 g of 2E4MZ. The solution wascooled in an ice bath, and 84.9 g of HTf was added thereto in a nitrogenatmosphere while stirring. The stirring was continued at roomtemperature overnight. The mixture was dried at 60° C. for 1 hour andthen at 110° C. for 6 hours under reduced pressure to remove ethanol andwater. The resulting 2E4MZ.HTf was in a liquid state for a while butsolidified in a few days. As a result of DSC, the melting point and theTg were found to be 6° C. and −91° C., respectively. The temperaturedependence of the ion conductivity of the 2E4MZ.HTf is displayed inFIGS. 1 and 4.

Example 7 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄)

In 10.3 g of 2E4MZ was added dropwise 5 ml of 98% sulfuric acid in anitrogen atmosphere while stirring. After the stirring was continued atroom temperature overnight, the mixture was dried at 110° C. for 6 hoursunder reduced pressure to remove water. The resulting 2E4MZ.H₂SO₄ was ina liquid state for a while but gradually solidified. In DSC, a samplewas maintained at 100° C. to once melt, cooled to −150° C., and againheated from −150° C. up to 100° C. The sample showed only a Tg with nopeak of crystallization or melting in both the cooling and the heatingthermogram. The Tg was −58° C. The temperature dependence of the ionconductivity of the 2E4MZ.H₂SO₄ is displayed in FIGS. 1 and 5.

Example 8 Mixture of 4-methylimidazole and sulfuric acid (4MI.H₂SO₄)

At 100° C. was melted 23.1 g of 4MI (from Aldrich), and 15 ml of 98%sulfuric acid was added thereto dropwise in a nitrogen atmosphere whilestirring. The stirring was continued at room temperature overnight. Themixture was dried at 110° C. for 6 hours under reduced pressure toremove water. The resulting 4MI.H₂SO₄ was in a liquid state for at leastone week. As a result of DSC, the melting point and the Tg were found tobe 29° C. and −62° C., respectively. The temperature dependence of theion conductivity of the 4MI.H₂SO₄ is shown in FIGS. 1 and 5.

Example 9 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄; molar ratio=9:1)

To 2.16 g of 2E4MZ was added 0.521 g of the 2E4MZ.H₂SO₄ obtained inExample 7 to give 2E4MZ.H₂SO₄ having a molar ratio of 9:1, whichmaintained a liquid state for more than six months.

Example 10 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄; molar ratio=3:1)

To 1.19 g of 2E4MZ was added 1.15 g of the 2E4MZ.H₂SO₄ obtained inExample 7 to give 2E4MZ.H₂SO₄ having a molar ratio of 3:1, whichmaintained a liquid state for more than six months.

Example 11 Mixture of 2-ethyl-4-methylimidazole and sulfiric acid(2E4MZ.H₂SO₄; molar ratio=3:2)

To 0.512 g of 2E4MZ was added 1.91 g of the 2E4MZ.H₂SO₄ obtained inExample 7 to give 2E4MZ.H₂SO₄ having a molar ratio of 3:2, whichmaintained a liquid state for more than six months.

Example 12 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄; molar ratio=2:3)

To 2.01 g of the 2E4MZ.H₂SO₄ obtained in Example 7 was added 0.484 g of98% sulfuric acid to give 2E4MZ.H₂SO₄ having a molar ratio of 2:3, whichmaintained a liquid state for more than six months.

Example 13 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄; molar ratio=1:3)

To 1.23 g of the 2E4MZ.H₂SO₄ obtained in Example 7 was added 1.16 g of98% sulfuric acid to give 2E4MZ.H₂SO₄ having a molar ratio of 1:3, whichmaintained a liquid state for more than six months.

Example 14 Mixture of 2-ethyl-4-methylimidazole and sulfuric acid(2E4MZ.H₂SO₄; molar ratio=1:9)

To 0.510 g of the 2E4MZ.H₂SO₄ obtained in Example 7 was added 1.93 g of98% sulfuric acid to give 2E4MZ.H₂SO₄ having a molar ratio of 1:9, whichmaintained a liquid state for more than six months.

Example 15 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=9:1)

To 30.1 g of 2E4MZ was added 7.92 g of the 2E4MZ.HTf obtained in Example6 to give 2E4MZ.HTf having a molar ratio of 9:1, which maintained aliquid state for more than seven months. The temperature dependence ofthe ion conductivity of the resulting 2E4MZ.HTf is shown in FIG. 4.

Example 16 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=7:3)

In 40 ml of ethanol was dissolved 43.0 g of 2E4MZ. The solution wascooled in an ice bath, and 25 g of trifluoromethanesulfonic acid wasadded thereto dropwise in a nitrogen atmosphere while stirring. Afterthe stirring was continued at room temperature overnight, the mixturewas dried at 60° C. for 1 hour and then at 110° C. for 6 hours underreduced pressure to remove ethanol and water to give 2E4MZ.HTf having amolar ratio of 7:3. The temperature dependence of the ion conductivityof the resulting 2E4MZ.HTf is shown in FIG. 4.

Example 17 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=7:3)

To 1.71 g of 2E4MZ was added 3.01 g of the 2E4MZ.HTf obtained in Example6 to give 2E4MZ.HTf having a molar ratio of 7:3, which maintained aliquid state for more than seven months.

Example 18 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=4:6)

To 4.01 g of the 2E4MZ.HTf obtained in Example 6 was added 1.17 g oftrifluoromethanesulfonic acid to give 2E4MZ.HTf having a molar ratio of4:6, which maintained a liquid state for more than six months.

Example 19 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=3:7)

To 3.05 g of the 2E4MZ.HTf obtained in Example 6 was added 2.35 g oftrifluoromethanesulfonic acid to give 2E4MZ.HTf having a molar ratio of3:7, which maintained a liquid state for more than six months.

Example 20 Mixture of 2-ethyl-4-methylimidazole andtrifluoromethanesulfonic acid (2E4MZ.HTf; molar ratio=2:8)

To 2.02 g of the 2E4MZ.HTf obtained in Example 6 was added 3.54 g oftrifluoromethanesulfonic acid to give 2E4MZ.HTf having a molar ratio of2:8, which maintained a liquid state for more than six months.

Example 21 Mixture of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H)

In an ice bath was cooled 6.62 g of 2E4MZ, and 5.78 g of methanesulfonicacid (from Aldrich) was added thereto dropwise in a nitrogen atmospherewhile stirring. Stirring was continued at room temperature overnight. Asa result of DSC, the resulting 2E4MZ.CH₃SO₃H was found to have a meltingpoint of 56° C. and a Tg of −63° C.

Example 22 Mixture of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H; molar ratio=7:3)

To 2.02 g of the 2E4MZ.CH₃SO₃H obtained in Example 21 was added 1.44 gof 2E4MZ to give 2E4MZ.CH₃SO₃H having a molar ratio of 7:3, whichmaintained a liquid state for more than seven months.

Example 23 Mixture of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H; molar ratio=4:6)

To 1.03 g of the 2E4MZ.CH₃SO₃H obtained in Example 21 was added 0.244 gof methanesulfonic acid to give 2E4MZ.CH₃SO₃H having a molar ratio of4:6, which maintained a liquid state for more than seven months.

Example 24 Mixture of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H; molar ratio=3:7)

To 2.00 g of the 2E4MZ.CH₃SO₃H obtained in Example 21 was added 1.25 gof methanesulfonic acid to give 2E4MZ.CH₃SO₃H having a molar ratio of3:7, which maintained a liquid state for more than seven months.

Example 25 Mixture of 2-ethyl-4-methylimidazole and methanesulfonic acid(2E4MZ.CH₃SO₃H; molar ratio=2:8)

To 1.09 g of the 2E4MZ.CH₃SO₃H obtained in Example 21 was added 1.53 gof methanesulfonic acid to give 2E4MZ.CH₃SO₃H having a molar ratio of2:8, which maintained a liquid state for more than seven months.

Comparative Example 1 Mixture of 2-ethylimidazole and sulfuric acid(2EI.H₂SO₄)

2EI (Aldrich) (27.1 g) was melted at 100° C., and 15 ml of 98% sulfuricacid was added thereto dropwise in a nitrogen atmosphere while stirring.The mixture was stirred at room temperature overnight, followed bydrying under reduced pressure at 110° C. for 6 hours to remove water.The resulting 2EI.H₂SO₄ was in a liquid state for a while but solidifiedin a few days. As a result of DSC, the melting point and the Tg werefound to be 50° C. and −64° C., respectively. The temperature dependenceof the ion conductivity of the 2EI.H₂SO₄ is shown in FIGS. 1, 4, and 5.The 2EI.H₂SO₄ largely reduced the ion conductivity in a temperatureregion at and below the melting point.

Reference Example 1 Mixture of imidazole and sulfuric acid/phosphoricacid (Im.H₂SO₄/H₃PO₄; molar ratio=2:1:1)

To 12.7 g of Im (from Sigma) was added dropwise 10.85 g of a 85%phosphoric acid aqueous solution and mixed. To the mixture was addeddropwise 5 ml of 98% sulfuric acid, followed by stirring overnight. Themixture was dried under reduced pressure at 80° C. for 1 hour and thenat 110° C. for 6 hours to remove water to give Im.H₂SO₄/H₃PO₄ having amolar ratio of 2:1:1. When allowed to stand overnight, theIm.H₂SO₄/H₃PO₄ solidified.

Reference Example 2

Thermogravimetric Analysis:

The results of thermogravimetric analysis on the acid-base mixtures ofExamples 1, 7, and 8 are shown in FIG. 2.

Reference Example 3

Thermogravimetric Analysis:

The results of thermogravimetric analysis on the acid-base mixtures ofExamples 2 and 3 and Comparative Example 1 are shown in FIG. 3.

Reference Example 4

Thermogravimetric Analysis:

The results of thermogravimetric analysis on the acid-base mixtures ofExamples 6, 21, 22, and 24, 2E4MZ, and methanesulfonic acid are shown inFIG. 6.

Reference Example 5

Thermogravimetric Analysis:

The results of thermogravimetric analysis on the acid-base mixtures ofExamples 7, 10, and 11 are shown in FIG. 7.

Reference Example 6

Thermogravimetric Analysis:

The results of thermogravimetric analysis on the acid-base mixtures ofExamples 7 and 8 and Comparative Example 1 are shown in FIG. 8.

INDUSTRIAL APPLICABILITY

The acid-base mixture according to the present invention is excellent inheat resistance and exhibits high ion conductivity without water or asolvent and is therefore useful as an ion conductor or a protonconductor in fuel cells, secondary batteries, electric double layercapacitors, and electrolytic capacitors.

The acid-base mixture of the present invention can be utilized in theabove-described applications as a polymer composite membrane obtained bysolution casting wherein the acid-base mixture and polymers are used ora polymer electrolyte membrane obtained by infiltrating the acid-basemixture into a porous polymer membrane.

1. An acid-base mixture comprising a base component and an acidcomponent, at least one of the base component and the acid componentcomprising at least two compounds, and the base component comprising atleast one compound represented by chemical formula (1):

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that at least one of them isa hydrocarbon group.
 2. The acid-base mixture according to claim 1,wherein the base component comprises at least one compound representedby chemical formula (2):

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that R¹ and R³ aredifferent.
 3. The acid-base mixture according to claim 1, having amelting point of 120° C. or lower or substantially no melting point. 4.The acid-base mixture according to claim 1, being an equimolar mixtureof the base component and the acid component.
 5. The acid-base mixtureaccording to claim 1, being liquid at room temperature.
 6. The acid-basemixture according to claim 1, wherein at least one of the basecomponents comprises 2-ethyl-4-methylimidazole.
 7. The acid-base mixtureaccording to claim 1, wherein at least one of the base componentscomprises 4-methylimidazole.
 8. The acid-base mixture according to claim1, wherein at least one of the base components comprises2-ethylimidazole.
 9. The acid-base mixture according to claim 1, whereinat least one of the acid components comprises an acid structurally freefrom a fluorine atom.
 10. The acid-base mixture according to claim 1,wherein at least one of the acid components comprises an inorganic acid.11. The acid-base mixture according to claim 10, wherein at least one ofthe acid components comprises sulfuric acid or phosphoric acid.
 12. Theacid-base mixture according to claim 1, being ion conductive.
 13. Theacid-base mixture according to claim 1, being proton conductive.
 14. Anion conductor comprising an acid-base mixture comprising a basecomponent and an acid component, the base component comprising a baserepresented by chemical formula (2):

wherein R¹, R², and R³ each represent a hydrogen atom or a hydrocarbongroup having 1 to 20 carbon atoms, provided that R¹ and R³ aredifferent.
 15. The ion conductor according to claim 14, wherein R¹ inchemical formula (2) is a hydrocarbon group having 1 to 20 carbon atoms.16. The ion conductor according to claim 15, wherein R¹ in chemicalformula (2) is a methyl group.
 17. The ion conductor according to claim15, wherein R² in chemical formula (2) is a hydrocarbon group having 1to 20 carbon atoms.
 18. The ion conductor according to claim 17, whereinR² in chemical formula (2) is an ethyl group.
 19. The ion conductoraccording to claim 14, wherein R³ in chemical formula (2) is a hydrogenatom.
 20. The ion conductor according to claim 14, wherein the basecomponent is 4-methylimidazole.
 21. The ion conductor according to claim14, wherein the base component is 2-ethyl-4-methylimidazole.
 22. The ionconductor according to claim 14, wherein the acid component is an acidstructurally free from a fluorine atom.
 23. The ion conductor accordingto claim 14, wherein the acid component is an inorganic acid.
 24. Theion conductor according to claim 23, wherein the inorganic acid issulfuric acid.
 25. The ion conductor according to claim 14, being aproton conductor.